The present invention relates to a thread-cutting device for an eyelet buttonhole sewing machine according to the preamble of claim 1.
An eyelet buttonhole sewing machine of the generic type became known as Exhibit 30 at the IMB Fair of 1997 in Cologne as Durkopp Adler Class 578-113221 E831 (recently designated as Cl. 578-113221 E1831), with which a stitch row is produced which is designed in the shape of a zig-zag and in alternating fashion as a single chain and double chain stitch seam. The eyelet buttonhole sewing machine includes rotatable sewing tools, including a rotatable looper support arranged below the support for the material to be sewn. It is possible to rotate the sewing tools and the looper support about an axis extending parallel to the longitudinal axis of the sewing needle. A thread cutter is arranged on the rotary looper support and is constructed with one thread-cutting tool for the needle thread and with another thread-cutting tool for the looper thread. Each thread-cutting tool can be moved independently of the other and may be operated via separate drives. Required movements for the thread-cutting tools are initiated by producing the drive connection from the thread-cutting tools to the drives arranged in a stationary fashion on the sewing machine frame in a defined rotary position of the looper support. Consequently, the thread-cutting device and/or the individual thread-cutting tools can be driven only subsequently, thus lengthening the machine cycle.
In principle, movement is initiated for a thread cutter of an eyelet buttonhole sewing machine as disclosed in DE-C 631 138.
Furthermore, disadvantages relating to application or use are entailed by the design described. Thus, even when laying out a seam, it is necessary for its end to always be aligned and/or arranged such that the looper support assumes the rotary position which permits thread cutting. In specific applications this leads to limitations which impair the appearance of a buttonhole produced. Furthermore, specific seam shapes, for example, a lacing hole, can cause limitations with regard to the position of the lacing hole relative to the actual part of the material to be sewn, and thus cause a reduction in quality.
There is further difficulty when it is intended to use the main cam plate for the thread-cutting device, as that plate rotates by only a few angular degrees. The eyelet buttonhole sewing machine may not receive the required drive movement at the end of the sewing operation.
The number of components which are required for the drive linkage to operate the thread-cutting device is also disadvantageous, including the required couplings for producing the drive connection in the defined position of the looper support relative to the machine housing and to the drives mounted thereon.